Field of the Invention:
The invention relates to development of axial flow steam turbine three
dimensional bladesformed of Nickel based super alloy that are required to
convert thermal energy of main steam entry temperature of 710°Cand
pressure of 304 bar into mechanical energy with high optimum stage
efficiency. The blades consisting of three dimensional air foil, root and
integral shroud are capable to withstand the demand of thermo mechanical stresses arising out of energy conversion of high enthalpy entry steam, at temperature of 710°Cand pressure of 304 barin which the blades formed of conventional ferriticsteel materials are not capableto withstand the thermo mechanical stresses.
Background of the Invention:
The developments in coal fired power stations have been vastly influenced by availability of materials, construction methods and economies. In the past few decades Concerns of Climate Change and availability of coal have become predominant to play role in the development of power station design. This has led to design of more efficient thermal cycles namely super critical and ultra-super critical than the conventional sub-criticalthermal cycle power plant. Above an operating live steam pressure of 221barand temperature of 373ºC, the cycle is called supercritical.The development of Supercritical power stations have increased efficiency figures to 38%- 40% from the range of 28%- 30% exhibited by typical single reheat power stations operating at 165 bar and 538°C. There is no standard definition for Ultra Super Critical power plants, and different parameters are being used in different parts of the world to categorize a power plant as a USC plant. However, as a common practice, any super-critical (SC) coal based power plant having main-steam temperature of above 600°C and steam pressure of above 275 bars has been considered as “Ultra Super Critical” (USC) as shown in Fig.1.

To compete with alternative methods for coal utilization like integrated gasification combined cycle (IGCC), supercritical and ultra-supercritical plants must achieve net plant efficiency > 40% (based on higher heating value, HHV of the fuel) on comparable capital cost of the plants. Supercritical technology has been developed and has reached maturity. Further increase in process temperature and pressure improves the cycle efficiency and reduces the carbon emissions. Efforts are underway to raise the steam parameters to values of 350 bar and 700-760°C. The plant with these parameters is popularly known as Advanced USC power plant.
However, no mature off-the-shelf technology exists for these Advanced Ultra Super Critical (AUSC) steam parameters. AUSC power plants, with 300 bar and 710°C steam cycle parameters, provide an excellent scope for improving gross plant efficiency to about 45%-47%, reducing coal consumption per mega watt of energy MW(e) and mitigating CO2 emissions by 10-15% compared to conventional sub-critical power plants. The energy security of the country can, therefore, be enhanced by extending the coal reserves by about 15-20%. In view of the above, an USC demonstration plant
of 800 MW(e) is to be established to operate with 304 bar and 710°C steam
parameters and to achieve the targeted plant efficiency of around 46%.
The steam turbine design starts with finalization of flow path based on the thermal cycle heat balance diagram. Steam enters the High Pressure Turbine at 304 bar and 710oC and expands through series of stationary and moving blades. The initial stages of turbine see high temperature and pressure and are subjected to thermo mechanical stresses. The blades made of conventional ferritic steels (Chrome-molybdenum-vanadium alloy steel) can’t withstand these stresses and require use of higher strength material.
The present invention relates to design of three dimensional blades made of Nickel based super alloy for axial flow steam turbine that are required to convert thermal energy of main steam with entry temperature of 710°C and pressure of 304 bar into mechanical energy with optimum stage efficiency.

India has well proven technology for the design of subcritical sets of various ratings. Significant percentage of the operational fossil power plants uses subcritical technology. The country has established know-how of supercritical technology for 660MW & 800MW rating sets, many of which are operational. This has led to the development of sub-critical and supercritical power plants, with new development of steam turbines for AUSC plant being taken-up.
Steam flow path design is an important and critical step in the turbine development process. Flow path design uses modular principles and also the standardized air foils which are pre-engineered and well evaluated through CFD, cascade testing at design and off-design condition for development of blades to the extent possible. The previously designed standardized blades used constant section air foil (cylindrical) with standardized root and shroud geometry. In order to improve the efficiency an improved cylindrical blades for axial turbine were attempted by Dr.A.L.Chandraker, Indian patent no.IN226263. The same author also pioneered the transonic blade profiles which can be used to develop 3D twisted blades for axial steam turbine e.g. US Patent no.US 7,175,393. Further, in an attempt to reduce the secondary flow losses, an improved three dimensional blade for axial turbine, Indian patent no.IN217153 was developed by the same author. All the above attempts were related to design of air foils applicable for the sub critical turbines wherein the blades formed of conventional ferritic steels, are capable to withstand the thermo mechanical stresses encountered during the operation. The present invention relates to blades that face higher temperature of upto 710°C and pressure of upto 304 bar(AUSC steam parameters) where conventional ferritic steel material can’t be used from the point of view of strength considerations.
Objects of the Invention:
The object of the invention is to develop three dimensional blades for axial flow steam turbine which can withstand the thermo-mechanical stresses at higher temperature and pressure.

Description of the Invention:
The present invention deals with development of turbine blades that are required to convert thermal energy of main steam with entry temperature of 710°C and pressure of 304 bar into mechanical energy.
A steam turbine shown in Fig.2 comprises of stationary blades (5) housed in casing and moving blades (6) mounted on rotor. Steam is admitted to the turbine from the steam generator, filling up the flow domain formed by casing shell (2) to expand in the bladed passages. In the process, it converts the thermal energy to mechanical energy and sets in the rotation of the rotor (3). Blades are the most important component of the turbine in determining the turbine efficiency (22) and consequently the heat rate of the power plant. Turbine efficiency (22) is defined as the ratio of actual heat drop in the turbine to ideal heat drop. Steam flow path and blade design is an important and critical step in the design cycle of steam turbine.
Blades are defined as components used for conversion of thermal energy to mechanical energy generated from three dimensional air foils (19) along with root (7) and shroud (8).
The design of steam flow path and blades consists of series of iterative processes, viz preliminary design, mean line design, meridional flow path design, air foil section design, stacking, design of root (7) and shroud (8), calculation of stresses in the blades, etc. Steam parameters and geometry boundary conditions are used in the preliminary design process to generate the flow path geometry and to select the optimal flow path parameters such as number of stages, geometrical dimensions, flow angles, heat drop distributions, stage reactions (23) etc. After preliminary design, mean line calculation is carried out to find the mass flow rate, velocities and velocity coefficients (losses) in the stages on mean line section. Mean line calculations are carried out on mid section and for long blades, it is necessary to take into account the span wise flow distribution. Hub to tip performance of the flow path is carried out using stream line equilibrium solver. The streamline

calculations are used to check the variation of kinematics at all the span wise sections and to make final geometry adjustments to meet the specified parameters like mass flow, flow angles, efficiency, etc.
The mean line and stream line design calculations finalize the overall flow path dimensions that include number of stages, axial length, blade angles, profile chord, profile throat, etc. The stream line calculation are carried out along the meridional plane at nine radial sections of the blade without finalizing the profile shape. This is accomplished by the task of profiling that generates aerodynamically perfect 2D section ofair foils at the radial stations. Profiling is a multi-parametrical task that requires control of several parameters at same time to achieve the optimum flow passage performance for the intended application. The profile development method breaks down the air foil into five distinct regions composed of the leading and trailing edge arcs, the suction and pressure side surfaces both modeled with higher order splines. In addition, Pressure and Mach number distribution, flow separation, channel width change, profile smoothness, profile loss, allowable stresses and manufacturability are some of the aerodynamic and structural parameters that influence the final shape of the air foil. The profile chord is optimized to ensure structural integrity during operation and manufacturability.
With the entry steam temperature of 710°Cand pressure of 304 bar, the blades of first stage onwards are subjected to creep and yield stress. The conventional blade materials like chrome-molybdenum-vanadium alloy steels (ferritic) for high temperature application have low creep rupture strength for 1,00,000 hours. With low strength material, design of air foil becomes infeasible. Hence, it is advantageous to use Nickel based super alloy with high creep rupture strength.
Three dimensional air foil is obtained by stacking the 2D air foils that are generated at several radial sections. The three dimensional shape of air foil depends on several factors like efficiency requirements, manufacturing capabilities and cost, etc. Cylindrical air foils are cheaper to manufacture but are less efficient. With the advent CNC machines, manufacturing capabilities

exist for any shape of 3D air foil. Hence, even though the cost of manufacture is high, three dimensional air foil with twist and lean are used to meet the efficiency requirement of the flow path.
Design of the three dimensional air foil is carried out by choosing the span wise optimum chord distribution. After profiling of hub, mean and tip sections, the profile sections are stacked with centroid & the three dimensional blades are generated and the stresses are checked with known material properties. Chord of air foil is adjusted slightly to meet the structural requirements. It is necessary to make the profiling of twisted blades to decrease the profile are from hub to tip and to gain the benefits of lower stress at the hub section with equivalent aerodynamic performance of the profile. Profile area depends on many geometrical parameters among which stagger angle is the most important. The angle depletion results in profile area decrease, the width and thickness increase, led to change in profile stiffness.
Constant relative pitch law is applied to all sections. The profiles are stacked using reverse twist law. Reverse twist law is defined as air foil twist method where the metal angle decreases from hub to tip. To satisfy the reverse twist law, a twist factor (26) applied from 0 to -1 in air foil.
The guide blades are designed using a typical twist factor which helps in even distribution of reaction and outlet flow angles in spanwise direction. Consequently, the rotor inlet flow angle becomes more uniform which makes the profiling of moving blades easier. It also helps in making the velocity fields evenly distributed which decreases the leakage flow and axial load. The guide blade local lean in the direction of rotation makes the radial reaction gradient lower thus decreasing the stage secondary losses. The applied local lean in the hub zone eliminates secondary losses in the hub area and also decreases the pressure and reactions gradient. Use of compound lean reduces thesecondary losses and increase the overall efficiency.

Smooth and aerodynamically perfect twisted blade is obtained by span wise controlling the distribution of geometrical parameters like stagger angle, unguided turning angle, relative blade length, etc. The three dimensional air foils of moving blades are generated with free vortex twist and compound lean to minimize incidence angle.
The two dimensional and three dimensional air foils are analysed for aerodynamic performance using Computational Fluid Dynamics (CFD). The aerodynamic performance parameters such as profile loss, profile loss coefficient, flow deflection, surface Mach number distribution are evaluated during 2D cascade analysis. Loss co-efficient ς was calculated using

where,

It is advantageous to obtain as low loss coefficient as possible by satisfying all the geometrical and permissible stress constraints.
To identify the presence of flow separations, if any, 3D CFD analysis was carried out. CFD analysis was carried out for the single bladed passage for the stage considering the labyrinth seal arrangement of the flow path. The results were comparable with stream line solver results.
The 3D air foil is positioned on the root platform. The axial width of the root is defined by the axial blade chord and some minimum distance at the upstream and downstream face. The root platform dimensions therefore depend on blade chord and stagger angle. Circumferentially grooved standard T root is used for simple and cost effective manufacturing process. T-roots are used based on chord size of the 3D profile and attached with the blade profile. High-low shroud is designed based on axial length at blade tip and hub for moving blade and guide blade respectively. The design of integrally shrouded blading is based on the use of rhombic T-roots and rhombic integral shrouds. The integral shrouds have two basic functions. Firstly they form a circumferential boundary to the steam path established by the blade profiles.

Thus they facilitate the design of optimal inter stage sealing with which leakage losses can be minimized. The second function is to provide support of the individual blades at their tips against each other. To keep this support as a closed 360° shroud ring under all operating conditions, the rhombic shrouds are elastically pre stressed.
Blade dimensioning is carried out in such a way, that the total stress, at the point of maximum stress, is below a material and temperature dependent limit value, which depends either on the yield strength or upon
creep-rupture strength for 1,00,000hours. Furthermore, the dynamic bending
stress resulting from steam forces which compound with the static stresses are lower than the allowable stress amplitude depending on material, temperature and geometrical shape. The guide blade profile experiences loads from aerodynamic forces (due to steam flow). The rotor blade profile experiences the loads from aerodynamic forces and centrifugal forces of the profile, root and shroud due to rotation. The flow field downstream of the cascade is not homogeneous and flow disturbances of the steam flow act as vibration inducing forces. The actual size of the unsteady forces cannot be calculated accurately, and hence entire load due to the aerodynamic forces is treated as dynamic load.
The centrifugal and aerodynamic forces and their resulting moments acting on the above sections generate normal, bending, shear and bearing stresses. The stresses are calculated at the critical sections of profile, root neck, root lug and rotor claw using formulas and finite element methods. The corresponding allowable stresses are calculated based on the material properties and factor of safety. The calculated stresses at the above critical sections are found to be lower than the allowable stresses for operating speed and over speed conditions. The structural design of blades is found to meet the operational requirements.
The embodiments of the invention are explained in detail with reference to the following drawings.

Brief Description of the Accompanying Drawings:
Fig 1 - Typical defining parameters of ultra super critical plants.
Fig 2 - Industrial steam turbine of prior art.
Fig 3 - Flow path of turbine of prior art.
Fig 4 - Flow path of AUSC HP turbine.
Fig 5 - Important geometrical parameters to generate the two
dimensional air foil.
Fig 6 - Optimized two dimensional air foil profile of a moving blade.
Fig 7 - Stress distribution on a 3D air foil from hub to tip of a blade.
Fig 8 - Stacked 2D air foils using centroid stacking method.
Fig 9 - Compound lean for moving blade hub section.
Fig 10 - Compound lean for guide blades.
Fig 11 - 3D air foil before and after applying the compound lean to guide
blades.
Fig 12 - Compound lean applied to moving blades.
Fig 13 - 3D air foil before and after applying the compound lean to moving
blades.
Fig 14 - Mach no. distribution for 2D cascade.
Fig 15 - Moving blade loading at mid-section from CFD analysis.
Fig 16 - Mesh of blade passage of stage with seals for CFD analysis.
Fig 17 - Velocity stream line plot of a stage blade passage with seals.
Fig 18 - 3D air foil.
Fig 19 - 3D moving blade.
Fig 20 - Critical sections for calculating the stresses in 3D blade.
Fig 21 - Finite element stress analysis results for a moving blade.
Detailed Description of different Embodiment of the Invention:
Fig 1 describes the typical defining steam parameters sub-critical, super¬critical and advanced ultra-super critical plants.
Fig 2 shows a solid model of industrial steam turbine according to prior art. The steam turbine comprises of casing or shell which houses the stationary blades, a turbine shaft with rotating blades, steam inlet and an

exhaust hood to exhaust the steam. Steam from steam generator enters through the inlet (1), expands in the flow direction of bladed passages. In the process, it converts the thermal energy to mechanical energy and sets in the rotation of the turbine shaft. The turbine shaft ultimately drives electric generator which is not shown in the figure.
Fig 3 shows flow path of a turbine according to prior art. The flow path comprises of stationary (guide) blades and moving blades. The blades consist of root and shroud which are integrally milled along with 2D/3D air foil. The root of the blade is used for inserting it into casing or turbine shaft depending on whether it is guide blade or moving blade. The integral shroud forms a circumferential boundary to the steam path established by the blade profiles. This facilitates the design of optimal inter stage sealing with which leakage losses can be minimized. It also provides support to the individual blades at their tips against each other.
Fig 4 shows the first three stages of HP Turbine flow path with steam entry temperature of 710°C and pressure of 304 bar. Steam enters the flow passage radially at angle, flows through the passages and exits axially. Steam expansion takes place in a predefined number of stages. It is advantageous to allow the expansion in a minimum number of stages provided certain aerodynamic and geometry variables are within permissible limits. In addition, the blades shall also withstand the thermo mechanical stress for desired period of operation. The geometry variables include turbine shaft diameter, blade aspect ratio, and minimum gauging angle, etc. The aerodynamic variables include isentropic velocity ratio, hub reaction, heat drop gradient, axial velocity gradient, number of stages, etc.
In order to withstand the thermo mechanical stress encountered by the blades a suitable material of construction isto be used. With the entry steam temperature of 710°C and pressure of 304 bar, the blades of first stage onwards are subjected to creep and yield stress. The conventional blade materials like chrome alloy steels for high temperature application have low

creep rupture strength for 1,00,000 hours. With such low strength material, design of air foil becomes infeasible. Hence, it is advantageous to use Nickel based super alloy with high creep rupture strength. Nickel based super alloy, IN617 is used as material of construction till the steam temperature drops below 600°C. As the steam expands, the steam entry temperature drops progressively and goes below 600°C after a predefined stage. Chrome alloy steels are used for the stages facing steam temperature of below 600°C. The air foil dimensions are finalized based on aerodynamic requirements and the allowable stress of material of construction. Hence the flow path consists of two distinctive parts, one consisting of Nickel based super alloy material and the other with chrome ally steel material.
Fig 5 shows the most meaningful geometric parameters that are required to generate an air foil. The air foil geometry is generated to match the inlet and exit velocity triangles that were finalized during the flow path design. Turbine blades have a lot of versatile requirements like aerodynamic quality, permissible stresses, manufacturability, etc and the process of air foil profile generation must meet all these requirements. The two dimensional air foil profiling process is carried out on plain section. Profile chord, leading edge and trailing edge radius and profile thickness distribution are the most important geometric parameters that influence the mechanical integrity and manufacturability of the blade. The aerodynamic quality depends on the pressure and velocity distributionin the inter blade channel to achieve minimum profile loss. The leading edge (LE) wedge angle sufficiently affects the profile form and stability on off-design (transient) operation. Its expansion leads to the profile area growth and reduces the profile sensitivity to flow angle variation on transient operation. The sizing of LE radius and LE wedge angle has to be compatible with the incidence angle considerations as well as 3D design aspects to reduce LE region loss and the corresponding secondary loss growth. Trailing edge (TE) wedge angle influences the flow in the unguided zone and the edge loss magnitude. These TE wedge angle and thickness need to be optimized to minimize the losses. The Stagger angle severely impacts the strength (area and stiffness), loss coefficient and channel

divergence characteristics of the profile. The stagger angle expansion leads to the shrinking of the profile width and area which result in the increase of the channel expansion (inter-blade passage divergence) ratio. In addition, inlet and outlet metal angle and trailing edge unguided turning angle, etc are the parameters that influence the aerodynamic quality of the air foil. Figure 5 shows five curves that describe the air foil. Smooth Mach no distribution across the profile and aerodynamic quality is obtained by adjusting the stagger angle, leading and trailing edge wedge angles and unguided turning angle of the profile. The exercise is carried out for all the nine radial sections of the blade and for all the stages.
Fig 6 shows the stress distribution on 3D air foil from hub to tip. The stresses on the blades are verified against the allowable stress. The allowable stresses are calculated depending on the operating temperature, either the
yield strength and average creep rupture strength for 10^5 hours are
identical. Hence the operating temperature of the blade decides whether allowable stress depends on creep rupture strength or yield strength.
Fig 7 shows dimensional air foil. While arriving at an optimal two dimensional air foil for the given inlet and exit velocity triangles, it is necessary to control the quality of flow in the channel to avoid flow separation, to achieve minimum profile loss and simultaneously verifying the allowable stress of the material.
Fig 8 shows stacked air foils using centroid stacking method.
Fig 9 shows compound lean applied to guide blades.
Fig 10 shows the 3D air foil before and after applying the compound lean to guide blades.
Fig 11 shows compound lean applied to moving blades.
Fig 12 shows the 3D air foil before and after applying the compound lean to moving blades.

Fig 13 shows the Mach no. distribution for 2D cascade and blade loading from CFD analysis. CFD simulation of 2D cascade is carried out using total pressure and total temperature with flow angle as inlet boundary condition and static pressure as outlet boundary condition. Outlet flow angle, inlet Mach number, outlet Mach number and pressure loss coefficient are calculated from CFD simulation. The inlet Mach number, outlet flow angle and pressure loss coefficient predicted by CFD simulation are in-line with stream line solver results carried out earlier.
Fig 14 shows moving blade loading at mid-section from CFD analysis.
Fig 15 shows the mesh of blade passage ofa stage with seals for CFD analysis. Total pressure and total temperature at inlet and mass flow rate at outlet are applied as boundary conditions and simulation is solved with shear stress transport turbulence model and converged to 5 e-5 resolution of the second order.
Fig 16 shows Velocity stream line plot of a stage blade passage with seals. The CFD results are compared with thermodynamic parameters obtained from stream line solver and they are in good agreement.
Fig 17 shows 3D air foil.
Fig 18 shows 3D moving blade.
Fig 19 shows the critical sections of the blade for evaluating the stresses. Loads considered are: Centrifugal load arising due to rotation aero dynamic load due to steam and load due to over speeding.
Fig 20 shows Finite element stress analysis results for a moving blade. The centrifugal and aerodynamic forces and their resulting moments acting on the above sections generate normal, bending, shear and bearing stresses. Formulas and Finite element methods were used to calculate the stresses at the critical sections of the blade. The allowable stresses are calculated separately for total stress, dynamic stresses and bearing stress at the operating and over speed. The stresses calculated at the critical sections of the blades are found to be lower than the allowable stresses.

We claim
1. Improved three dimensional blades consisting of 3D air foil, root and
shroud for axial flow steam turbine, which can withstand the thermo-
mechanical stresses at higher temperature and pressure, comprising:
two dimensional air foils with leading edge (28) with inlet metal angle (15); leading edge radius (10) and leading wedge angle (13) and inlet metal angle (15) and trailing edge (29) with exit metal angle (31), trailing edge radius (11) and trailing edge wedge angle (16); a suction face (32) between the leading edge (28) and trailing edge (29) on the opposite side of pressure face (33) and axial chord (9) formed between the lines connecting the leading edge (28) and trailing edge (29); wherein the three dimensional blades are formed of two dimensional air foil as described above by varying cross section,
by choosing the span wise chord distribution and controlling the geometrical parameters like stagger angle (17), unguided turning angle (18), and then stacking the air foil section with centroid, wherein the profiles of air foil are stacked by using reverse twist law, the blades being designed using typical twist factor (26) and compound lean (20) for decreasing the stage secondary losses.
2. The blades as claimed in claim1, wherein the leading edge radius (10)
varying from 0.015 to 0.08 of axial chord (9), trailing edge radius (11)
varying from 0.005-0.02 of axial chord (9), leading edge wedge angle
(13) varying from 60°-82°, trailing edge wedge angle (16) varying from
2°-10°, stagger angle (17) varying from 20°-27°, trailing edge unguided
turning angle (18) varying from 5°-160°.

I
3. The blades as claimed in claim 1, wherein the stage reaction (23) varying
from 20-40% and with a minimum blade nieght of 10mm.
4. The blades as claimed in claim 1, wherein the two dimensional air foil have a profile loss coefficient (21) varying from 0.031 to 0.016 hub to tip.
5. The blades as claimed in 1, wherein the two dimensional air foils are stacked with a twist factor (26) varying from 0 to 1 and applying compound lean (20) from hub to tip for reducing the secondary flow losses.

6. The blades as claimed in 1, wherein the two dimensional air foils are stacked with minimum incidence loss (24) and applying compound lean (20) from hub to tip for reducing the secondary flow losses.
7. The blades as claimed in claim 1, wherein the blades with labryinth seals can convert the energy with more than 90% total to total isentropic efficiency (22).
8. The three dimensional blades as claimed in claim 1, wherein the blades are capable to up to handle mass flow rate varying from 291 kg/s to 565 kg/s.
9. The blades as claimed in claim 1, which the blades are capable for a
minimum of one lakh hours of opeartion under different loading
conditions (25) for a steam temperature range of 710°C -600°C.